The present invention relates to a method for the preparation of a fused silica glass body co-doped with a rare earth element and aluminum. More particularly, the invention relates to a method for the preparation of a fused silica glass body doped with a rare earth element as the principal dopant and aluminum as a co-dopant and useful as a base material of optical fiber lasers and the like.
Fused silica glass doped with a rare earth element is used as a material of optical fiber lasers, light amplifiers, optical sensor elements and the like by utilizing the unique optical properties exhibited by the rare earth element as the dopant. It is also known that the optical performance of such fused silica glass doped with a rare earth element can be further improved when the glass is co-doped with aluminum. In an optical fiber of fused silica glass doped with erbium used in light amplifiers at a wavelength band of 1.55 .mu.m in the optical communication system, for example, co-doping with aluminum has an effect to expand the wavelength region for amplification to give a possibility of simultaneously amplifying light signals transmitted at two or more different wavelengths.
Rare earth-doped fused silica glass can be prepared by the so-called MCVD method disclosed, for example, in Japanese Patent Kohyo 63-501711, according to which a chloride of the rare earth element is heated and vaporized at a high temperature and the vapor of the rare earth chloride is mixed with the vapor of a silicon compound and introduced into an oxyhydrogen flame in which the vapors are flame-hydrolyzed to be converted into the respective oxides to be deposited to form a porous glass body which is subsequently sintered and vitrified into a transparent glass body. A problem in this MCVD process is that difficulties are sometimes encountered in exactly controlling the feed rate of the rare earth chloride vapor relative to the vapor of the silicon compound due to the relatively low vapor pressure of the rare earth chloride, especially, when the doping level with the rare earth element should be very low, which is usually 500 ppm by weight or less, although this method may be applicable to a fused silica glass body doped with aluminum alone because of the relatively high vapor pressure of aluminum compounds as compared with rare earth compounds.
Alternatively, Japanese Patent Publication 58-3980 proposes a method for the preparation of a rare earth-doped fused silica glass body, according to which a silicon compound alone is subjected to flame-hydrolysis in an oxyhydrogen flame to produce fine particles of silica which are deposited to form a porous silica glass body and this porous body is then soaked and impregnated with a solution of a rare earth compound followed by drying and sintering to effect vitrification of the body into a transparent fused silica glass body. This method is indeed advantageous in the versatility that the method is applicable to any rare earth compounds having a relatively low vapor pressure irrespective of the doping level with the rare earth element.
The matter is not so simple, however, when this solution-impregnation method is applied to co-doping of a fused silica glass body with a rare earth element and aluminum, presumably, due to the difference in the affinity of the rare earth element and aluminum with the solvent of the solution and with the surface of silica forming the porous body. For example, the concentration of the aluminum compound is usually higher in the vicinity of the surface of the porous silica body than in the core portion due to the stronger adsorption of the aluminum compound than the rare earth compound and migration of the aluminum compound from the core portion to the surface layer by the capillary phenomenon in the course of drying. This problem is serious when the doping level with aluminum, which is usually in the range from 1000 to 10,000 ppm by weight, is so high that accumulation of the aluminum compound in the surface layer may cause inhibition of vitrification of the silica in that portion.